Immunocytochemistry with antibodies against the calcium binding proteins parvalbumin (PV) and calbindin (CaBP-28kD) was used to study horizontal cells in the macaque monkey retina. Both morphological types (H1 and H2) are stained by PV immunocytochemistry. Horizontal cells in the centre of the fovea are described for the first time. From a minimum of 250/mm2 at the foveal centre, total horizontal cell density rapidly increases to a peak of 23,000/mm2 in an annulus of 0.6 mm radius around the fovea. Horizontal cell density then drops continuously to approximately 800–1000 cells/mm2 in peripheral retina. The density ratio of cones to horizontal cells is 1.5 in central and 4 in peripheral retina.
The cones were stained with antibodies against CaBP-28kD. Thus the spatial correlation between the cone inner segments and the cone pedicles could be measured to show that they are in perfect register. There is no reorganization of the spatial array of cone outer segments to produce chromatically specific connections between cone pedicles and horizontal cells.